Title: “Tabletop Ultrabright Kiloelectronvolt X-Ray Sources from Xe and Kr Hollow-atom States”Abstract: Today, two x-ray sources, x-ray free electron lasers (XFEL) and synchrotron radiation sources are producing extremely bright x-rays. However, a number of applications of these facilities, such as medical and industrial, have been hampered by their size, complexity, and cost. We have developed two tabletop ultrabright-keV x-ray sources, one from a Xe hollow-atom state and the other from a Kr hollow-atom state, with a unique characteristic that makes them complementary to the extreme-light sources. The Xe (M) x-ray source operates at 1.2-1.6 nm wavelength while the Kr (L) x-ray source operates in 600-800 pm wavelength. The system is mounted upon 3 optical-tables (5´x12´) with two KrF amplifiers at a repetition rate of 0.1 Hz. Experimentally estimated lower-bound brightness values for both Xe and Kr x-ray sources is 1026 photons s-1mm-2mrad-2. This is greater than the average brightness (1022 photons s-1mm-2mrad-2 0. 1% BW) for the available x-ray free electron lasers and synchrotron x-ray sources. Further, both x-ray sources show convincing evidence for directionality, which is one of the characteristics of lasers.